1. Field of the Invention
The present invention relates to a method for forming dielectric films and, in particular, to a method for forming dielectric films including metal nitride silicate suitable for use as a high permittivity constant gate dielectric films in a semiconductor device.
2. Description of the Related Art
A semiconductor device such as MOS (Metal Oxide Semiconductor) transistors have been accelerated in channel length reduction because of implementation of high operating speed. However, much reduction in the channel length lowers the electrostatic capacity of gate dielectric films, thus causing malfunction of a transistor's switching operations. Accordingly, thinning the gate dielectric films provide electrostatic capacity capable of switching operations of transistors. Conventionally, as a material of the gate dielectric films of a MOS transistor, a silicon dioxide film (SiO2) capable of good interface characteristics in a simple manufacturing process has been used. However, with a decrease in the film thickness of the gate dielectric films up to approximately several nm, gate leaks by tunnel current generate in large quantity, thus causing a problem of high power consumption. As means for solving this problem, there has been proposed a method for forming gate dielectric films using a material having higher relative permittivity constant than relative permittivity constant ∈r=3.9 of SiO2. The dielectric films made of such a material is referred to as high permittivity constant dielectric films (high-k dielectric films). The high-k dielectric films can increase, as the material has higher relative permittivity constant, the film thickness to a required degree to obtain the same electrostatic capacity as that of a silicon dioxide film, thereby inhibiting an increase in leakage current.
As one candidate of materials of high-k dielectric films, for example, metallic dioxide such as ZrO2 and HfO2 has been proposed. Conventionally, as a method for depositing such a metallic dioxide on a surface of a substrate, there has been known MOCVD (Metal Organic Chemical Vapor Deposition) disclosed in Japanese Patent Application Laid-Open No. 2004-140292 (US counterpart U.S. Pat. No. 7,105,362, US Patent Application Publication 2006/0008969). According to the MOCVD method, a metallic complex raw material is put in a raw material tank, heated with a heater to become a liquid state and carrier gas is flowed into the raw material tank to vapor raw material and is transferred into a reaction chamber. The raw material transferred to the reaction chamber is deposited onto a surface of a heated substrate, thus implementing film formation.
However, the film formed according to the MOCVD method is a film including many impurities such as carbon or hydrogen derived from an organic raw material. There has occurred the following problem: the film formed according to the MOCVD method tends to generate a large amount of leakage current by an influence of residual impurities. As another method for depositing, on a substrate, metallic dioxide including few impurities derived from a raw material like the MOCVD method, a sputtering method has been known. According to the sputtering method, for example, by making noble gas such as Ar ionized by plasma collide with a surface of a target, atoms constituting the target are rejected and deposited on a surface of the substrate. The content of impurities in the deposited film is less than that according to MOCVD. By performing sputtering using a plurality of targets containing different elements, films having various compositions can be easily formed. For example, using a target made of metal and a target made of Si, the targets are sputtered at the same time to deposit a film containing metal and silicon. By oxidizing such a film, a metal silicate film can be easily formed.
Referring now to FIGS. 5A through 5E, a conventional method for manufacturing high-k dielectric films to which a sputtering method is applied will be described below. Reference numeral 201 denotes a substrate containing single-crystal silicon, reference numeral 202 denotes a silicon dioxide film, reference numeral 203 denotes a film containing metal and silicon, reference numeral 204 denotes a metal silicate film and reference numeral 205 denotes a metal nitride silicate film.
In a step of FIG. 5A, first, contaminants on a surface of the substrate 201 are removed according to a known RCA cleaning method to expose silicon atoms on a surface of the substrate 201.
Next, in a step of FIG. 5B, a surface of the substrate 201 is oxidized to form a silicon dioxide film 202. As a method for oxidizing the substrate 201, any method of thermal oxidizing method, radical oxidizing method and the like may be used if the method provides a good silicon interface.
Next, in a step of FIG. 5C, by performing sputtering with a target containing metal and a target containing silicon, a film 203 containing metal and silicon is deposited on a surface of the silicon dioxide film 202.
Further, in a step of FIG. 5D, the film 203 containing metal and silicon is oxidized by oxygen radical to form a metal silicate film 204. Oxidization of the film 203 containing metal and silicon can be implemented by a radical oxidizing method capable of low-temperature oxidization without use of thermal oxidizing method. Generally, metal oxide film and metal silicate film is lower in thermal stability than silicon dioxide film. In the thermal oxidizing method for performing oxidization at a high temperature, the metal silicate film in an amorphous state may be crystallized. The crystallized metal silicate film tends to generate a leakage current. Accordingly, the radical oxidization capable of low-temperature oxidization is suitable. As a unit for generating oxygen radical, any generating method of plasma excitation, optical excitation or the like may be used if active oxygen radical can be formed. Otherwise, in performing sputtering, sputtering while introducing oxidization reaction gas such as oxygen, that is, reactive sputtering may be used. In this case, the atom sputtered from the target is oxidized and is deposited on the substrate as an oxide, thus omitting an oxidizing step after sputtering.
Next, in a step of FIG. 5E, the metal silicate film 204 is nitrided to form a metal nitride silicate film 205. Thermal stability is improved by nitriding the metal silicate film 204 and diffusion of impurities from an electrode formed on the gate dielectric films can be inhibited. In nitriding the metal silicate film 204, nitrogen plasma can be used. Using nitrogen plasma enables the metal silicate film to be nitrided at a lower temperature than for thermal nitriding. In thermal nitriding, however, nitrogen atoms are diffused close to a surface of the substrate 201 or close to an interface with silicon, which may cause performance degradation such as degradation of channel mobility. On the other hand, plasma nitriding enables only proximity to a surface of the silicate film to be selectively nitrided, thus providing resistance to causing such degradation.
Referring to FIGS. 6A through 6D, description will be made on another conventional embodiment in manufacturing high-k dielectric films to which a sputtering method is applied.
In a step of FIG. 6A, contaminations on a surface of the substrate 201 are removed by a known RCA cleaning method to expose silicon atom on a surface of the substrate 201.
Next, in a step of FIG. 6B, a film 203 containing metal and silicon is deposited on a surface of the substrate 201 by performing sputtering using a target made of metal and a target made of silicon.
Further, in a step of FIG. 6C, a metal silicate film 204 is formed by oxidizing the film 203 containing metal and silicon. At that time, upon oxidizing species' reaching the substrate 201, silicon on a surface layer portion of the substrate is oxidized to form a silicon oxide film 202 between the substrate 201 and the metal silicate film 204.
Further, in a step of FIG. 6D, the metal silicate film 204 is nitrided to form a metal nitride silicate film 205.
Hence, it is possible to form high-k dielectric films having the same structure as the one obtained by the method described in FIGS. 5A through 5E.
On the other hand, Japanese Patent Application Laid-Open No. H11-168096 discloses a method for directly forming high-k dielectric films on a silicon layer without degradation in the characteristics of the high-k dielectric films. After the high permittivity nitride films are formed on the silicon layer, the high permittivity nitride films are oxidized to be changed into high permittivity oxide films. However, this can form high permittivity oxide films, but cannot form a metal nitride silicate film.
Oxygen is diffused very fast in the film containing metal and silicon and, according to a method illustrated in FIGS. 5A through 5E and 6A through 6D, it is easy for oxygen to penetrate through the film containing metal and silicon and diffuse up to the ground in a step for oxidizing the film containing metal and silicon. Hence, the substrate containing the ground silicon is oxidized to cause an increase in a film thickness of the silicon dioxide film and an insulation film containing the silicon dioxide film (an increase in film thickness), thus causing a problem of difficulty in obtaining a film thickness of each film as designed. Conventionally, because controllability of a film thickness of the silicon dioxide film and that of an insulation film containing the silicon dioxide film were poor, countermeasures therefor have been demanded.
Further, problems in nitriding the metal silicate film will be described below.
The concentration of nitrogen atom in plasma-nitriding an Hf silicate film formed by using Hf (hafnium) as a metal material was measured by XPS (X-ray-induced photoelectron spectroscopy). The composition between the atoms of Hf atom and Si atom included in the Hf silicate film was in a ratio 1:1. In comparison, the concentration of nitrogen atom was similarly measured in nitriding the silicon dioxide film under the same nitriding conditions. As the result, the surface density of nitrogen atom introduced into the silicon dioxide film is 6.5×1015 atoms/cm2, while the surface density of nitrogen atom introduced into the Hf silicate film was 1.2×1015 atoms/cm2. In spite of nitriding under the same conditions, the amount of nitrogen atoms in the Hf silicate film was smaller than that in the silicate oxide film. To increase the amount of nitrogen atoms introduced into the Hf silicate film, nitriding conditions such as prolonging the nitriding time have been studied, but is understood more difficult to introduce nitrogen atoms into the Hf silicate film than the silicon dioxide film. This is not only seen in the metal silicate film using Hf as metal but as is the case with the metal silicate film using another metal such as Zr. This is as in the case of a thermal nitriding method instead of a plasma nitriding method. Further, it is understood that nitriding is more difficult as the composition ratio of metal atoms in relation to Si atoms in the silicate film is higher.
Because it has conventionally been difficult to nitride the silicate film having a high composition ratio of metal in a high concentration, various measures therefor have been demanded.